In the dental field one of the most sought after alloys is one to which porcelain can be applied by fusion and which will have complete adhesion throughout the temperature range and other conditions to which the alloy and porcelain will be subjected during processing and use.
Among the many well recognized problems in applying low-fusing porcelain to a dental alloy to provide exterior porcelain surfaces thereon is that the coefficient of expansion of the alloy and the porcelain should be completely matched and compatible so that on cooling the porcelain will not check, crack or separate from the metal alloy.
Also, with precious metal alloys such as alloys of yellow gold, platinum and palladium in fractional percentages it has not been possible to obtain a true bond between the alloy and the porcelain. Hence various alternatives have been suggested for adding to a precious metal alloy other metal elements which at porcelain fusing temperatures would provide on the surface of such alloy bonding oxides.
Among the metals proposed to be added to provide such bonding oxides were copper, cobalt and silver. It has been found, however, that the oxides of copper, cobalt and silver are undesirable for dental purposes from the standpoint of esthetics for the reasons that copper oxide turns porcelain green; cobalt oxide turns porcelain blue; and silver oxide turns porcelain yellow.
Another disadvantage of a yellow gold precious metal alloy from the standpoint of application of low-fusing porcelain is that yellow gold melts at 1940.degree. F. and low-fusing porcelain fuses at 1800.degree. F. As a consequence the temperature differential between the yellow gold and the fusing porcelain was too close and the yellow gold was unable to maintain its rigidity during the fusing operation.
In order to strengthen yellow gold and to improve its rigidity for the purpose of fusing porcelain thereto, it has heretofore been proposed to add platinum and palladium to provide a precious metal alloy. In order to maintain the yellow color of gold, however, it has been found that the white metals (platinum and palladium) should not exceed 10% of the alloy composition.
In view of the high cost of gold and its high specific gravity, which approximates 19, efforts have been made to employ palladium as a substitute for yellow gold. Even though palladium has the white color, it was thought desirable to employ palladium in dental alloys because of the other physical and workability characteristics of such alloy. Also, as a substitute for yellow gold it was found that palladium could be combined with silver, nickel and other elements to provide an alloy suitable for porcelain application which would have the high temperature melting point necessary and would also have the rigidity of an alloy structure necessary for porcelain application.
Among the prior art patents generally relating to the subject matter of this invention are:
U.S. Pat. No. 3,819,366 in the name of Michel Katz and entitled "Dental Alloy", wherein the precious alloy for use in dental frames on which ceramic coverings or acrylic coverings are formed consists of 8-76 weight percent palladium and 0.2-18 weight percent indium, 0-15 weight percent zinc, and other trace metals in amounts no greater than 13/4 weight percent.
Schaffer U.S. Pat. No. 3,928,913, entitled "Palladium Alloy for Ceramic Bonding" discloses a dental casting alloy consisting essentially of 40-60 percent of a precious metal component selected from the group consisting of palladium and mixtures of palladium and platinum wherein the platinum is in the amount of up to 12 percent of the alloy but not in excess of 25 percent of the palladium content; 20-59 percent of a non-precious metal component selected from the group consisting of 20-50 percent cobalt and mixtures of 20-50 percent cobalt and up to 25 percent nickel wherein the nickel content does not exceed the cobalt content; and a modifier selected from the group consisting of 1-8 percent indium, 1-3 percent tin and 1-8 percent of mixtures of tin and indium wherein the tin does not exceed 3 percent.
U.S. Pat. No. 3,929,474, dated Dec. 30, 1975 for "Tarnish Resistant Silver Based Dental Casting Alloy Capable of Bonding to Porcelain" in the name of Clyde E. Ingersoll, which discloses and claims an alloy to which porcelain may be directly fused, -- the alloy composition comprising: 35-60% Pd, at least about 0.5-7% of one member of the group consisting of Cr, Fe, In and Sn and from about 0 to 5% of the group consisting of Si, Ni, Co, Ta, and Ti, and the rest Ag.
For completeness of disclosure it should be noted that Schaffer U.S. Pat. No. 3,928,913 also mentions molybdenum in the amount of 2-8 percent as a desirable additive and that nickel in the amount of 10-20 percent desirably comprises a portion of the non-precious metal component.